1. Field of the Invention
The present invention relates to a pellicle for lithography that is used as a debris shield for a lithographic mask when producing a liquid crystal display panel or a semiconductor device such as an LSI or a ULSI, a mounting method therefor, a pellicle-equipped mask, and a mask.
2. Description of Related Art
In the production of a semiconductor such as an LSI or a VLSI or the production of a liquid crystal display panel, a pattern is formed by irradiating a semiconductor wafer or a liquid crystal substrate with light through an exposure master plate; if debris is attached to the exposure master plate used here, since the debris absorbs the light or bends the light, there are the problems that the replicated pattern is deformed, the edge becomes rough, or the background is stained black, thus impairing the dimensions, quality, appearance, etc. The ‘exposure master plate’ referred to in the present invention is a general term for lithographic masks and reticles.
These operations are usually carried out in a clean room, but even within a clean room it is difficult to always keep the exposure master plate clean, and a method is therefore employed in which a pellicle that allows exposure light to easily pass through is adhered to the surface of the exposure master plate to act as a debris shield.
The pellicle is basically constituted of a pellicle frame and a pellicle film stretched over the frame. The pellicle film is formed from nitrocellulose, cellulose acetate, a fluorine-based polymer, etc., which allows exposure light (g rays, i rays, 248 nm, 193 nm, etc.) to easily pass through. The pellicle film is adhered by coating the upper end part of the pellicle frame with a good solvent for the pellicle film and air-drying or by means of an adhesive such as an acrylic resin, an epoxy resin, or a fluorine resin. Furthermore, in order to mount an exposure master plate, a lower end part of the pellicle frame is provided with a pressure-sensitive adhesion layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicon resin, etc. and a reticle pressure-sensitive adhesive protecting liner for the purpose of protecting the pressure-sensitive adhesive layer.
The pellicle is installed so as to surround a pattern region formed on the surface of the exposure master plate. Since the pellicle is provided in order to prevent debris from becoming attached to the exposure master plate, this pattern region and a pellicle outer part are separated so that dust from the pellicle outer part does not become attached to the pattern face.
In recent years, the LSI design rule has shrunk to sub-quarter micron, and accompanying this the wavelength of an exposure light source is being shortened, that is, instead of g lilne (436 nm) and i line (365 nm) from the hitherto predominant mercury lamp, a KrF excimer laser (248 nm), an ArF excimer laser (193 nm), etc. are being used. As shrinkage advances, the flatness required for the mask and silicon wafer becomes more strict.
A pellicle is affixed to a mask in order to shield a pattern from debris after the mask is completed. When a pellicle is affixed to a mask, the mask flatness can sometimes change. When the mask flatness is degraded, as described above there is a possibility that a problem such as defocusing will occur. Furthermore, when the flatness changes, a pattern shape drawn on the mask changes, and this brings about the difficulty that problems occur with the precision of superimposition of the mask.
There are several factors involved in the change in mask flatness due to a pellicle being affixed, but it has been found that the largest factor is the flatness of the pellicle frame.
The pellicle is affixed to a mask via a mask pressure-sensitive adhesive present on one side of the pellicle frame; when the pellicle is affixed to a mask, the pellicle is usually pressed against the mask with a force of on the order of 20 to 30 kgf. In general, the flatness of the mask is a few μm or less as a TIR value, and is 1 μm or less for a state-of-the-art mask, and the flatness of the pellicle frame is generally on the order of a few tens of μm, which is large compared with the mask. Because of this, when the pellicle is affixed to the mask, the flatness of the mask might change due to unevenness of the frame. Here, it might be thought that by increasing the flatness of the pellicle frame so that it is as high as the flatness of the mask it would become possible to reduce the change in flatness of the mask.
The pellicle frame is generally formed from an aluminum alloy. With regard to a pellicle frame for semiconductor lithography, the width is on the order of 150 mm, the length is on the order of 110 to 130 mm, and it is generally formed from a pellicle frame bar having a rectangular cross-section. A frame is generally prepared by cutting out a pellicle frame shape from an aluminum alloy plate or extrusion-molding an aluminum material into a frame shape, but since the width is as narrow as on the order of 2 mm and it is easy to deform, it is not easy to produce a flat frame. Because of this, it is very difficult to make the pellicle frame have a flatness of the same degree as that of the mask.
In order to prevent deformation of the mask due to deformation of the pellicle frame, JP-A-2008-65258 (JP-A denotes a Japanese unexamined patent application publication) discloses a pellicle in which the thickness of a mask pressure-sensitive adhesive via which the pellicle is affixed to a mask is at least 0.4 mm, and a pellicle for which the modulus of elasticity at 23° C. of the mask pressure-sensitive adhesive is no greater than 0.5 MPa.